'Official stability' means we will not alter the terrain shape (that includes the structure of caves) unless it is unavoidable as part of a bugfix.
For non-mgv6 mapgens the biome system is separate and defined by MTGame, and is not stable, however changes are likely to be subtle and the worst that can happen is a few straight-edged biomes.

Mgv6 has been officially stable since 2012. Since mgv6 biomes are hardcoded into the engine the biomes are also stable.

Mgv7 terrain has been officially stable since 0.4.16 when it was made the default mapgen, however the optional floatlands (disabled by default) are not stable yet, maybe they will be for 0.5.0.

For mgv5:
Terrain surface shape is identical or as close as it can be. The eased 3D noise new mgv5 uses may create a slightly different shape to the original 3D noise implementation.
The 3D noise tunnels are identical in shape.
Old mgv5 only had these tunnels and had random water and lava sources added underground. New mgv5 doesn't have these sources, instead it adds the 'large caves' from mgv6, which often contain water or lava, these were considered a better way to include underground liquids.
Also there is an option (default enabled) for the giant caverns deep underground.

Mgv7 surface and underground is stable. The floatlands are not but they are disabled by default, they also seem quite well tuned so they might become stable at 5.0.0.

Mgflat in flat mode is stable. The only uncertainty about stability is the 'lakes' and 'hills' options but there seems no reason to alter those.

Mgvalleys and mgv5 seem near-stable. maybe for 5.0.0.

We're very hesitant about officially announcing a mapgen as stable because that is a big commitment that rules out certain types of improvement. However mapgens are often near-stable long before being 'officially' stable.
If you wait for official stability you'll miss out on good stuff for years. It's better to accept that very occasionally there may be a discontinuity in terrain.

paramat wrote:and that mod could possibly also smooth the transitions.

Yes, such a mod already exists. However, detecting borders is slightly more complicated. It would need a on_generated callback which then loads the surrounding mapblocks to check whether there's a large change in terrain height in order to flatten it there. In addition to the height change, the biome might also have changed - which has then to be spread a bit to make the transition look acceptable.
It's much work to implement this - and the mod would cause additional lag for each new chunk to ensure a continuous terrain.

I intend to have mgv5 and mgflat announced as officially stable for 5.0.0.
Mgvalleys is still somewhat of a mess, that i am slowly sorting out, so won't be for a while.
Mgcarpathian i might add a feature to (that has support from the mapgen creator), and is quite new, so again not for a while.

Now, we only state which mapgens (or parts of mapgens) are 'highly unstable'.
Everything else should be considered as increasingly stable with time.
Now we recognise that stability is a smoothly varying thing, instead of 2 states: 'unstable' and 'stable'.
It is always possible, if it is absolutely necessary, that something may change, even in v6.
Any changes are likely to be minor, at worst, terrain being discontinuous between previously generated world and newly generated world.
Any mapgen that makes it into a stable release will be supported from then onwards.

Currently, only v7 floatlands are highly unstable. I am not happy with their generation, i might tune the parameters or even rewrite their generation method.

Last edited by paramat on Thu Jun 06, 2019 17:44, edited 1 time in total.